1. Field of the Invention
The present invention relates generally to magnetic recording media, and more particularly to magnetic recording disks having discrete servo tracks formed by laser ablation of a carbon underlayer.
Thin film magnetic recording disks generally comprise a disk substrate having a magnetic layer and a number of underlayers and overlayers deposited thereon. The nature and composition of each layer is selected to provide desired magnetic recording characteristics, as generally recognized in the industry. An exemplary present day thin film disk is illustrated in FIG. 1 and comprises a non-magnetic disk substrate 10, typically composed of an aluminum alloy. An amorphous nickel phosphorous (Ni--P) underlayer 12 is formed over each surface of the disk substrate 10, typically by plating and is subsequently polished and sometimes texturized prior to deposition of the additional films. The Ni--P layer is hard, and imparts rigidity to the aluminum substrate. Alternatively, glass and other non-metallic materials are now used to form highly rigid disk substrates. A second underlayer in the form of a chromium ground layer 14 is formed over the Nii--P layer 12, typically by sputtering, and a magnetic layer 16 is formed over the ground layer 14. The magnetic layer 16 comprises a thin film of ferromagnetic material, such as a magnetic oxide or magnetic metal alloy. Usually, a protective layer 18, such as a carbon film, is formed over the magnetic layer 16 and a lubricating layer 20 is formed over the protective layer.
The presence of the Ni--P underlayer 12, together with the chromium ground layer 14, has been found to improve the recording characteristics of the magnetic layer 16. In particular, the chromium ground layer formed over a Ni--P layer provides enhanced coercivity and reduced noise characteristics. Such improvements are sometimes further enhanced when the Ni--P underlayer is treated by mechanical texturing to create a roughened surface prior to formation of the chromium ground layer. The texturing may be circumferential or crosswise, with the preferred geometry depending on the particular composition of the cobalt-containing magnetic layer.
The outer carbon protective layer 18 serves a very different purpose. This protective layer has been found to greatly extend the life of magnetic recording media by reducing disk wear. Carbon has been shown to provide a high degree of wear protection when a thin lubrication layer 20 is subsequently, applied.
Such magnetic recording disk constructions have been very successful and allow for high recording densities. As with all successes, however, it is presently desired to provide magnetic recording disks having even higher recording densities. One method for increasing the a real density on rigid magnetic disks involves patterning the surface of a thin film disk to form discrete data tracks. Such "discrete track media" typically include surface geometry data which are utilized by the hard disk drive servo mechanism, allowing specific recording tracks to be identified, and providing feedback to improve the accuracy of read/write head tracking.
Such discrete track media, however, suffer from their own disadvantages. The surface patterns of discrete track media have generally been imposed using standard lithographic techniques to remove material from the magnetic recording layer or by creating recessed zones or valleys in the substrate prior to deposition of the magnetic material. In the former case, the magnetic recording material is etched or ion milled through a resist mask to leave a system of valleys which are void of magnetic material. In the latter case, the magnetic film, subsequently applied, is spaced far enough away from the recording head that the flux from the head does not sufficiently "write" the magnetic medium. Servo track information can be conveyed by the difference in magnetic flux at the boundary between the elevatored patterns and the valleys. However, the boundary signals have at most 50% of the amplitude of conventionally recorded data. Additionally, fabrication of production quantities of discrete track media has remained problematic, due in part to the expense of the required lithographic processes.
It has recently been proposed to produce discrete track magnetic recording media through the use of a pre-embossed rigid former magnetic disk. The surface pattern would be directly imprinted on a substrate using a stamping process. Thin film magnetic recording layers could then be sputtered over the patterned surface of the substrate, producing discrete track media having a continuous magnetic layer extending over both the elevated regions and the recessed zones or valleys. Unfortunately, the stamping process inherently requires elevated temperatures and pressures, and is susceptible to warping. Additionally, the intricate pattern must be first precisely imprinted on the stamping plate, as the stamping process will reproduce any flaws or contaminants present on the stamping plates. Development of discrete track media using such a stamping process is particularly problematic, as each test pattern requires production of an expensive stamping tool.
For these reasons, it would be desirable to provide an improved method for producing a discrete track servo pattern for discrete track media. It would be particularly desirable if such a method provided the accuracy and reproducability of lithography, but did not involve multiple process steps or the complex, dedicated tooling required for stamping. It would be best if such a method enhanced the improvements to the magnetic recording characteristics available using the conventional underlayers, magnetic recording layers, and overlayers of high density magnetic recording media.
2. Description of the Background Art
The production of discrete track media and other magnetic recording media having patterned surfaces were described by S. E. Lambert et al. in Beyond Discrete Tracks: Other Aspects of Patterned Media, JOURNAL OF APPLIED PHYSICS, Vol. 69, 8:4724-26, Apr. 15, 1991. Each of the patterned media described were produced by sputter etching or ion milling a magnetic recording layer through a resist mask. The resist mask was written with an electron beam, as is known in the lithographic arts.
The production of discrete track media through the use of a pre-embossed rigid magnetic disk was described by D. Dericotte, et al., in Advancements in the Development of Plastic Hard Disks With Pre-embossed Servo Patterns, CORPORATE RESEARCH LABORATORIES, SONY CORPORATION. The disk is produced using an injection molding process between two stamping plates. The plates containing the media surface pattern are produced using lithographical techniques.
Recording media having a selectively laser-textured surface and methods for their production are described in U.S. Pat. Nos. 5,062,021, and 5,108,781, respectively. A laser system for texturing a substrate, Ni--P layer, or a magnetic recording layer is also disclosed.